1. Field of the Invention
The present invention relates to a surface acoustic wave device used for, for example, a resonator and a band pass filter. In particular, the present invention relates to a surface acoustic wave device including an IDT electrode and a silicon oxide film disposed on a LiNbO3 substrate and utilizing Rayleigh waves.
2. Description of the Related Art
Band pass filters used for RF stages of cellular phones and other electronic devices must be broadband and have good temperature characteristics. Therefore, a surface acoustic wave device in which an IDT electrode is disposed on a piezoelectric substrate made of a rotated Y-cut x-propagation LiTaO3 substrate or a rotated Y-cut x-propagation LiNbO3 substrate and a silicon oxide film arranged so as to cover the IDT electrode has been used. With this type of piezoelectric substrate, the temperature coefficient of resonant frequency has a negative value. Therefore, to improve the temperature characteristic, the silicon oxide film having a positive temperature characteristic of resonant frequency is arranged so as to cover the IDT electrode.
However, with this structure, when the IDT electrode is made of general-purpose Al or an alloy primarily including Al, a satisfactory reflection coefficient of the IDT electrode cannot be obtained. Consequently, there is a problem in that a ripple is likely to occur in the resonant characteristic.
To solve the above-described problems, WO2005-034347 discloses a surface acoustic wave device in which an IDT electrode primarily made of a metal having a density greater than that of Al is disposed on a piezoelectric substrate made of LiNbO3 having an electromechanical coefficient K2 of at least about 0.025, a first silicon oxide film having a film thickness substantially equal to the thickness of the electrode is disposed in a region other than the region in which the IDT electrode is disposed, and a second silicon oxide film is laminated so as to cover the electrode and the first silicon oxide film.
In the surface acoustic wave device described in WO2005-034347, the density of the above-described IDT electrode is specified to be at least about 1.5 times greater than the density of the first silicon oxide film, and the reflection coefficient of the IDT electrode is thereby satisfactorily increased and a ripple which occurs in the resonant characteristic is suppressed.
In WO2005-034347, the Rayleigh wave is used, Au, Cu, and other suitable materials may be used for the IDT electrode, a configuration in which when the electrode is made of Cu, the film thickness thereof is specified to be about 0.0058λ to about 0.11λ is disclosed, and in particular, if the film thickness is specified to be about 0.0058λ to about 0.055λ, the electromechanical coefficient K2 of the Rayleigh wave can increase. Furthermore, a LiNbO3 substrate represented by Euler angles of (0°±5°, 38°±10°, 0°) is disclosed, and a configuration in which the film thickness of the second silicon oxide film is specified to be within the range of about 0.15λ to about 0.4λ is shown, where the wavelength of the surface acoustic wave is λ.
In recent years, the frequencies used in surface acoustic wave devices have increased. Consequently, the pitch of electrode fingers of the IDT electrode has been reduced and the width dimension of the electrode finger itself has been reduced. As a result, the wiring resistance increases and the loss in the surface acoustic wave device increases.
An increase in film thickness of the electrode is sufficient to reduce the loss in the surface acoustic wave device. However, as described in, for example, WO2005-034347, in the past, with surface acoustic wave devices utilizing the Rayleigh wave, the film thickness of the IDT electrode made of Cu is specified to be a maximum of about 0.11λ. The reason for this is believed to be that if the film thickness of the IDT electrode made of Cu is increased to greater than about 0.11λ, the response of the SH wave significantly increases, and a large spurious response occurs between the resonant frequency and the anti-resonant frequency. Therefore, in WO2005-034347, when the IDT electrode is made of Cu, the thickness thereof is specified to be within the range of about 0.0058λ to about 0.11λ, and preferably about 0.058λ to about 0.055λ.
Consequently, when higher frequencies are used, the pitch of the electrode fingers is reduced, and the width dimension of electrode finger is reduced, the film thickness of the electrode cannot be sufficiently increased. Therefore, the wiring resistance increases and the loss increases.
Furthermore, as described above, with the surface acoustic wave device including the first and the second silicon oxide films, the temperature characteristic of resonant frequency is improved by the silicon oxide films. However, there is a problem in that the characteristic varies because of variations in film thickness of the silicon oxide films.